Electronic repulsion for bonded electrons—
l.p.-l.p. > l.p-b.p > b.p-b.p.
Nitrogen is more electronegative than Hydrogen, it attracts shared pair of electron and electron density accumulates near N, to minimize repulsion between bond pairs, angle increases
For NF3:
Bond pairs are near fluorine, this would lead to more b.p.l.p. repulsion and to minimize it, bond angle decreases. Although; NH3 and NF3 have same hybridization but F is more electronegative than H, the amount of p character in the N-F bonds in NF3, will be higher than in N-H bonds in NH3. Hence, bond angle in NF3 is smaller.
